Enhancement: Customizable feedback link

We've added options and documentation for customization of the `href`, `ariaLabel` and `description` of the feedback link in the topbar. See https://owncloud.dev/clients/web/getting-started/ for documentation.

https://github.com/owncloud/web/issues/6702
https://github.com/owncloud/web/pull/6761
https://owncloud.dev/clients/web/getting-started/
